The present invention relates to a source of charged particles and more particularly to a charged particles source in which when a needle-shaped tip is applied with a negative potential relative to the electrode facing the tip, it functions as a field emission cathode (electron source) and when applied with a positive potential functions as a field ionization ion source.
The field emission cathode (electron source) and the field ionization ion source have been used in totally different equipment although they have the same shape of tip. This is because the polarity of the charged particles generated by the electron source is opposite to that of the ion source and the current characteristics of the two sources are different. In the micro-fabrication in the range of micron and sub-micron, it is desirable that the features of electron beam and ion beam are both applied to lithography and fabrication and that the ion beam lithography and the micro-doping by ion implantation be performed in the vacuum using electron beams while enabling the operator to watch the whole process. To meet these requirements the best solution is to employ the tip that can be used both as an electron source and an ion source.
The tip material that has widely been used as the electron as well as ion source is a tungsten (W). This material, however, has problems. That is, when the tungsten tip is used as an ion source, the chemical reaction between the tip surface and the hydrogen gas makes it impossible to use it for a long period of time. When on the other hand the tungsten tip is used as an electron source, the emitted electrons ionize the residual gas molecules in the vacuum space and these ions are accelerated, as opposed to the electrons, to bombard the tip thereby increasing instability of field emission current. Thus, the material used as electron source must have a sufficient strength to undergo bombardment of ions. The tungsten, however, cannot be said to meet this requirement.